Generally in the production of pulp suitable for formation into paper, wood or other raw cellulosic fibrous material, is subjected to chemical digestion in a pulping liquor to form a pulp of the cellulosic fibrous material. In the present invention, the pulping liquor contains or consists of sodium sulphide. The pulp thereafter is subjected to brightening and purification operations in a bleach plant.
The spent pulping liquor from the digestion usually is subjected to a series of recovery and regeneration operations to recover unused pulping chemicals and to provide fresh pulping liquor.
A widely used pulping process is the Kraft process. While the present invention will be described hereinafter with particular reference to Kraft mills, the process of the invention also is applicable to other pulp mill operations which utilize sodium sulphide or in which aqueous sodium sulphide is produced as an intermediate product. Typical of such additional processes are high yield pretreatment Kraft, polysulfide, alkafide and sodium-based sulphite processes. In the conventional Kraft process, raw cellulosic fibrous material, generally wood chips, is digested, by heating, in a pulping liquor, known as white liquor and containing sodium sulphide and sodium hydroxide as the active pulping chemicals to provide a pulp and spent pulping liquor, known as black liquor. The black liquor is separated from the pulp by washing in a brown stock washer and the pulp then is passed to the bleach plant for brightening and purification operations.
The black liquor then is passed to the recovery and regeneration system in which the black liquor first is concentrated, usually by evaporation, and the concentrated black liquor is burned in a furnace to yield a smelt containing sodium carbonate and sodium sulphide. A sodium-and sulphur-containing compound, generally sodium sulphate, is added to the black liquor generally prior to feed of the concentrated black liquor to the furnace, although such sodium- and sulphur-containing compounds may be added at any other convenient location, such as to the white liquor prior to the digestion step, to make up sodium and sulphur values lost from the recovery system.
The smelt is dissolved in water to yield a raw green liquor which then is clarified to remove undissolved solids. The clarified green liquor, containing dissolved quantities of sodium carbonate and sodium sulphide, is causticized with slaked lime whereby the sodium carbonate is converted to sodium hydroxide and calcium carbonate mud. The resulting liquor is white liquor which then may be recycled to the digestion step to provide at least part of the pulping liquor.
Bleach plant operations generally involve a sequence of brightening and purification steps, together with washing steps. The brightening steps involve the use of bleaching agents, and in the present invention, at least one of the brightening steps preferably involves the use of at least one chlorine-containing bleaching agent. Such chlorine-containing bleaching agents include chlorine, chlorine dioxide, chlorine monoxide and sodium hypochlorite.
The purification step generally involves treatment with a sodium hydroxide solution, and usually is known as a caustic extraction step. In some instances, the bleaching and caustic extraction steps may be combined, for example, using the so-called "oxygen bleaching" operation. Where oxygen bleaching is used, however, it is used in this invention preferably in combination with one or more bleaching operations using chlorine-containing bleaching chemicals.
A particular bleaching operation which has been employed involves an initial bleaching of the pulp with an aqueous solution containing chlorine or a mixture of chlorine dioxide and chlorine, an intermediate washing, a caustic extraction using aqueous sodium hydroxide solution, a further washing, a bleaching with an aqueous solution of chlorine dioxide, another washing, a further caustic extraction using sodium hydroxide, an additional washing, a final bleaching with chlorine dioxide solution and a final washing. This is the so-called CEDED operation. The present invention will be described with particular reference to this procedure, although other procedures may be employed, such as, the use of an aqueous solution containing approximately 100% chlorine dioxide in the first bleaching step.
The above-described CEDED operation may be carried out using the so-called "Dynamic Bleaching" process claimed in Canadian Pat. No. 783,483. In this process, pulp treating solutions are passed successively through a mat of fibers in which the fibers are maintained relatively stationary with respect to each other. Washing steps, except for a washing after the last step of the bleaching and purification sequence may be omitted.
The spent wash waters from bleach plants generally have been discharged to water bodies, such as streams, rivers, lakes and oceans, without any attempt to recover chemicals therefrom, although in some instances solid particle recovery operations have been made. One of the main reasons that no attempt has been made to recover these chemicals is because they are very dilute and of small value. The bleach plant also produces spent bleaching liquor and spent caustic extraction effluents. These effluents have objectionable colour and are toxic and harmful to aquatic and marine biota and polluting since they contain fibers and materials consuming oxygen present in the water. It is desirable to avoid such environmental pollution and hence avoid the discharge of these effluents from the mill.
Due to the use of chlorine-containing bleaching chemicals and sodium-containing purification agents, the spent wash water contains substantial quantities of sodium chloride. In addition, when the spent bleaching liquor effluents and the spent caustic extraction liquor effluents are mixed at least part of the residual chlorine and soda values combine to form sodium chloride. In the present invention, the normally discharged effluents, namely, the spent wash water, the spent bleaching chemicals and the spent caustic extraction liquor preferably are mixed to provide a single bleach plant effluent stream, known as BPE and the bleach plant effluent is not discharged to waste.
The quantity of chlorine-containing bleaching agents and the quantity of sodium hydroxide used as caustic extraction liquor preferably are balanced to provide about one atom of sodium for each atom of chlorine, whereby these chemicals form sodium chloride and thereby to provide a neutral effluent. The equivalence of sodium and chlorine atoms in the bleach plant effluent is preferred so that the overall sodium inventory of the mill thereby remains unchanged. In the CEDED sequence when chlorine or mixtures of chlorine dioxide and chlorine in which the proportions of available chlorine provided by chlorine dioxide is low, is used in the first stage an amount of sodium hydroxide solution in excess of that required for extraction must be added to match the chlorine atoms present. If no excess is added, only about 40 to 50% of the first chlorination stage filtrate can be recovered to match the stoichiometric equivalent of sodium atoms used in the extraction. Where, however, the available chlorine is provided predominantly by chlorine dioxide, typically above about 70%, the quantities of sodium and chlorine atoms are susbstantially equivalent and hence it is preferred to employ the latter sequence.
Sodium chloride present in the bleach plant effluent also may arise from sodium chloride present in the pulp when it is introduced to the bleach plant. Such sodium chloride may be present where the logs are floated in sea water prior to formation of wood chips therefrom. The use of sea-borne logs also causes there to be present in the black liquor sodium chloride from washing of the pulp in the brown stock washer. In addition, if brackish water is used to provide the bleach plant wash water, sodium chloride again is present to the bleach plant effluent.
Alkali metal salts may be introduced to the pulp mill system, from other sources, such as from the cellulosic fibrous material itself. Further, sodium chloride may be introduced as a contaminant of make up chemicals, provided from a natural source or formed as a product of a chemical process. For example, the make up chemical may be sodium sulphate from a natural source contaminated with sodium chloride or sodium hydroxide formed by electrolysis of sodium chloride solution and contaminated with sodium chloride.
In the present invention, the bleach plant effluent preferably is added to the spent liquor recovery and regeneration operation, and in this way this effluent is retained within the mill. It has been proposed previously in Canadian Pat. No. 832,347 and U.S. Pat. No. 3,698,995 to reduce the environmental problems of bleach plant effluents by utilizing the spent wash waters to wash the pulp in the brown stock washer. The use of the spent wash water in this manner reduces the overall water requirement of the mill. In the present invention, it is preferred to use the bleach plant effluent, consisting of a mixture of spent wash waters, preferably provided by countercurrent washing as described in Canadian Pat. No. 832,347 and U.S. Pat. No. 3,698,995, spent bleaching chemicals and spent caustic extraction liquor to wash the pulp in the brown stock washer. By operating in this manner, the water requirement is reduced and in addition a liquid effluent-free pulp mill is provided.
In a particular manner of carrying out such counter-current washing in a CEDED sequence in which the pulp is washed after every step, there is a completely countercurrent flow of liquors, namely spent bleaching liquors, spent caustic extraction liquors and wash water, with respect to the flow of pulp through the bleach plant. In such an operation, fresh water or white water is passed into contact with the pulp following the last bleaching stage, the spent wash water from this washing being mixed with spent chlorine dioxide solution from the last bleaching stage. The resulting mixture is split into two streams, the bulk being used to wash the pulp from the last caustic extraction stage, and the remainder being used to mix with spent caustic extraction effluent from the last caustic extraction stage, the mixture being used partially to mix with effluent from the washing step after the intermediate bleaching stage.
The remainder of the mixture is used as wash water to wash the pulp from the intermediate bleaching stage. Spent bleaching chemical from this stage is mixed with the aqueous material resulting from the last mixing, the resulting material, representing the combined effluents from the subsequent steps of the bleach plant, being used partially to wash the pulp from the first caustic extraction stage and partially to wash the pulp from the first bleaching stage. The spent wash water from the washing of the pulp from the first caustic extraction stage is mixed with spent caustic extraction liquor from the first caustic extraction and the mixture is divided into two streams, one of which represents an alkaline effluent and the other is used as wash water for the pulp from the first bleaching stage. The spent wash water from this stage is mixed with spent bleaching chemical from the first bleaching stage to provide an acid liquor, part of which constitutes an acid bleach plant effluent.
Another part of the acid liquor may be used to mix with the pulp received from the brown stock decker to provide the water required to bring the pulp to the consistency necessary in the first bleaching stage. A further part of the acid liquor is used as the aqueous medium for gaseous chlorine used in the first bleaching stage. In this way, a portion of the acid liquor, constituted by the latter two parts may be recycled in the first bleaching step.
The acid spent bleaching chemical liquor is mixed with the alkaline caustic extraction liquor to provide the bleach plant effluent which is used to wash the pulp in the brown stock washer.
The bleach plant effluent may be introduced at other stages of the recovery and regeneration operation, as desired. Further, the bleach plant effluent may be split into two or more streams which are introduced at different locations of the recovery and regeneration operations, for example, to provide the "weak wash" water or to dilute concentrated white liquor.
The quantity of sodium chloride present in the bleach plant effluent varies depending on the bleaching sequence which is employed. In a typical procedure where a mixture of chlorine dioxide and chlorine are utilized in the first stage of a CEDED sequence the quantity of sodium chloride may vary between about 120 and 160 lbs/ton of pulp depending on the proportion of chlorine dioxide used. Typically, when the total available chlorine in the first stage is provided 70% by chlorine dioxide and 30% by chlorine, the quantity is about 120 lbs/ton of pulp.
The introduction of the bleach plant effluent to the pulping liquor recovery and regeneration operations closes the whole system to liquid effluents and hence sodium chloride cannot be purged from the mill by way of discarded bleach plant effluent. The sodium chloride remains unconverted by the black liquor recovery steps and hence would build up in the system upon continued recycle of regenerated white liquor.
In order to prevent such build up and at the same time utilize the concepts of an effluent-free pulp mill, it is essential to remove sodium chloride from the system. Such removal of sodium chloride should be such that the other valuable components utilizable as or convertible into pulping chemicals are not removed from the system along with the sodium chloride and hence the normal chemical balance and economy is retained. In addition, it is preferred to remove a quantity of sodium chloride from the mill equivalent to the amount introduced to and/or produced within the mill, typically about 120 lbs./ton pulp.
The present invention in a preferred aspect therefore is directed to the removal from the mill of sodium chloride introduced thereto with bleach plant effluent. However, in its broadest aspect, the invention is directed to the removal from the mill of sodium chloride present therein from any source, such as one or more of the sources mentioned above.
In Canadian Pat. No. 915,361 and U.S. Pat. No. 3,746,612, it is proposed to remove sodium chloride from pulp mill recovery and regeneration procedures by concentration of white liquor, preferably by evaporation, in order to precipitate and remove sodium chloride from the white liquor. This procedure is satisfactory and may be utilized to remove the desired quantity of sodium chloride from the mill.
The sodium chloride also is present in the green liquor prior to causticization together with sodium sulphide and sodium carbonate. The causticization operation normally carried out on the green liquor therefore is exposed to the sodium chloride-content of the green liquor, which may cause a lowering in the efficiency of the causticization operation and hence an increase in the quantity of uncausticized sodium carbonate present in the white liquor and a decrease in the alkalinity content (NaOH + Na.sub.2 S) of the white liquor as compared with causticization of green liquor to obtain white liquor in the absence of sodium chloride. In addition, the presence of the sodium chloride in the green liquor may cause corrosion problems in the recausticizer. It, therefore, would be of advantage if sodium chloride could be removed from the system prior to the causticization.
It is not possible, however, to use the process outlined in the above-mentioned Canadian Pat. No. 915,361 and U.S. Pat. No. 3,746,612 to precipitate sodium chloride directly from the green liquor, due to the presence therein of substantial quantities of sodium carbonate which would be precipitated.